Google is your friend - have a hunt for titles like "practical antenna handbook" and "VHF UHF antenna", assuming you're talking about the EU 446 MHz band take the dimensions from a nice-looking aerial for close to that frequency in one of the books that has some gain and scale it up a little i.e. 446/432* (*or whatever frequency it was designed for) times as big, only element lengths though, don't worry about finding e.g. 9.3765875865675 mm diameter tubing!

432 MHz may also be known by wavelength, 70cm, and as most aerials' individual elements are in the 1/4 to 5/8 wavelength range that should give you an idea of the scale of the finished item (and how accurately you need to work, to the mm or better would be good).
Bear in mind that gainy beam aerials are VERY directional once you get past 3 or 4 elements, collinear have less gain but cover the full 360 around you. ANYTHING you make (if you're careful) will way outperform the rubber ducky that comes fitted as standard to PMR radios...

You may need to modify the radio if it doesn't have an aerial socket, not beyond what's possible if you can wield a soldering iron, usually getting the radio apart and back together takes longer than the soldering...

Ive had great success with home brewed boomerangs on 2M. In its simplest form a dipole made from 2 x copper pipes. The coax runs inside the centre element and emerges at the bottom, so it loo,s like a vertical with the coax connected at the bottom. I'm currently using one for part lessons and bicycle work, made Hampshire from Northampronshire on FM with a Bumfeng UV5R, so the performance is beyond question.

To be honest, and analyser is great, but the most useful tool for fiddling with antennas is a simple UHF VSWR meter, as it's the comfort blanket = you press the PTT, and it tells you instantly if the radio is happy by showing you the VSWR, and then as you nibble little bits off the elements you SEE the change. Analysers like the one I have show you curves and readings, but a real meter connected to your radio makes things so much simpler. PMR446 is only a smidge above the ham band, so all the antennas for that band work pretty well without any major changes, just a very small loss of efficiency. I have a commercially made antenna cut for 449MHz and that works perfectly well on 70cm - and of course 446 in the middle. The reality is that the days of radios dying from poor VSWR is pretty long gone - most low power transmitters sense the poor VSWR and ease back on the power rather than dying on you - mainly because people use all sorts of rubbish antennas, and if they died every time a load wasn't 50~Ohms, the manufacturers would get lots of warranty returns, which they don't!